FIG. 1 illustrates the network architecture of an Evolved Universal Terrestrial Radio Access Network (E-UTRAN), where a Mobility Management Entity (MME) is connected with an evolved Node B (eNB) via an S1-MME interface; and the eNB functions as an access network and communicates with a User Equipment (UE) via an air interface. Each UE attached to the network is served by an MME which is referred to a serving MME of the UE. The S1-MME interface which provides the UE with control plane services including mobility management and bearer management functions. A Serving Gateway (S-GW) is connected with the eNB via an S1-U interface, and each UE attached to the network is served by an S-GW which is referred to as a serving S-GW of the UE. The S1-U interface provides the UE with user plane services, and user plane data of the UE is transmitted between the S-GW and the eNB over an S1-U General Packet Radio Service (GPRS) Tunnel Protocol (GTP) bearer.
The traditional scheme of single-layer network coverage by a macro eNB has been unable to satisfy a constantly growing demand of subscribers for a data service rate and a service capacity, so layered network deployment has been introduced to address the problem in that some low-power eNBs (referred below to as local eNBs in the femto, pico, relay or other forms) are deployed in a hotspot area, a home indoor environment, an office environment or other small-coverage environments, to provide small coverage (i.e., small cell), thereby splitting a cell so as to enable an operator to provide a subscriber with a service at a higher data rate and a low cost.
FIG. 2 illustrates the layered heterogeneous network architecture including a Local eNB and a Micro eNB, where the Macro eNB provides underlying coverage, and the Local eNB (i.e., a small cell) provides hotspot coverage; and there is a data/signaling interface (which can be a wired or radio interface) between the small cell and the macro eNB.
Since a small cell controlled by the local eNB has such a small coverage area that there are a small number of UEs served by the cell, a UE connected with the local eNB tends to be provided with a higher quality of service, e.g., a higher service rate, a link with a higher quality, etc. When a UE connected with the macro eNB is proximate to the cell controlled by the local eNB, the UE can be switched to the small cell served by the local eNB; and when the UE is remote from the small cell served by the local eNB, the UE needs to be switched to a cell controlled by the macro eNB to keep wirelessly connected.
In order to address the problem of mobility in the heterogeneous network, the scheme of network deployment with bearer separation has been introduced so that Radio Resource Control (RRC) connections of the UE are maintained only at the macro eNB, and all or a part of data bearers thereof are transferred to the local eNB for transmission.
FIG. 3 illustrates such a network architecture with bearers being separated, under this network architecture, all of Signaling Radio Bearers (SRBs) of the UE are maintained at the macro eNB, and all or a part of Data Radio Bearers (DRBs) are transferred to the local eNB for transmission, where an interface represented in a dotted line exist only if a part of the DRBs are bearer separated.
FIG. 4 illustrates another such a network architecture with bearers being separated that all or a part of DRBs can be transferred to the local eNB for transmission.
In the heterogeneous network, there may be several small cells in the macro coverage area, and these small cells may overlap in coverage area with each other. If the UE is located in an area covered by more than one of the small cells, then the UE may receive signals concurrently from the more than one small cell, and these signals may interfere strongly with each other. In order to the address the problem of interference, the Coordinated Multiple-Point Transmission/Reception (CoMP) mechanism may be introduced between the small cells to suppress the interference so as to improve the quality of receiving the signals by the UE.
The CoMP refers to that a plurality of transmission points separate in geographical location cooperate to participate in transmitting data to a UE or receiving jointly data transmitted by the UE. Generally the plurality of transmission points are eNBs of different cells or a plurality of Remote Radio Heads (RRHs) controlled by the same eNB. Interference between the plurality of transmission points can be lowered effectively by coordinated scheduling, pre-coding, joint transmission, etc., between the plurality of transmission points to thereby improve the throughput of a user covered in the cooperation area, particularly an edge user covered by the coordinating points.
Related terms in the CoMP are defined as follows:                Points: a set of transmit antennas using the same address in geographical location, where different sectors of the same site are different points.        A CoMP serving cell: a cell transmitting CoMP-related Physical Downlink Control Channel (PDCCH) scheduling information.        A CoMP resource management set: the UE makes Channel State Reference Signal-Reference Signal (CSI-RS)-based Reference Signal Received Power (RSRP) and/or Reference Signal Received Quality (RSRQ) measurement, and reports a measurement result to the CoMP serving cell, and the CoMP serving cell manages a CoMP measurement set based upon the RSRP and/or RSRQ measurement result by determining the measurement CoMP set and determining a point to be added to or deleted from the CoMP measurement set.        A CoMP measurement set: the CoMP measurement set is determined from the measurement result by the CoMP resource management set or is determined from a Radio Resource Management (RRM) measurement result of mobility measurement or may be even determined from a Sounding Reference Signal (SRS) using channel reciprocity in a Time Division Duplex (TDD) system. The UE performs CSI measurement in the CoMP measurement set and reports CSI measurement results to the CoMP serving cell to determine a CoMP cooperating set and the CoMP transmission points.        A CoMP cooperating set: a set of points participating directly or indirectly in transmitting or receiving data. The CoMP cooperating set may or may not be transparent to the UE. The CoMP cooperating set is determined from the CSI measurement results of the CoMP measurement set and can further be determined from the SRS using the channel reciprocity for the TDD system.        CoMP transmission points: one point or a set of points in the CoMP cooperating set to participate directly in transmitting data to the UE. The CoMP transmission points are determined from the CSI reported for the CoMP measurement set or determined using the channel reciprocity for the TDD system.        CoMP reception points: one point or a set of points in the CoMP cooperating set to receive data of the UE. The CoMP reception points can be determined from the SRS.        
The CoMP is further categorized into Downlink (DL) CoMP and Uplink (UL) CoMP based on different transmission directions as described below respectively:
(1) DL CoMP:
For the DL CoMP, the UE receives scheduling information carried on the PDCCH from only one transmission point but can receive data concurrently from one or more transmission points. The DL CoMP is categorized into Joint Processing (JP) and Coordinated Scheduling/Beam-forming (CS/CB).
The JP is further categorized into Joint Transmission (JP) and Dynamic Point Selection/Muting (DPS). The JT refers to that all or a part of the points in the CoMP cooperating set transmit data concurrently to one or more UE; and the DPS refers to that only one point in the CoMP cooperating set transmits data concurrently for the UE at any time. The selected transmission point can vary over time. The JT and the DPS can be applied jointly.
The CS/CB is characterized in that there is only one such point in the CoMP cooperating set that has traffic data of the UE and that will transmit the data to the UE, whereas the other points will feed scheduling or beam-forming information back to the transmission point. For example, time and frequency resources utilized for transmission can be coordinated between the points in the CoMP cooperating set in the CS to thereby lower interference as much as possible. The JP and the CS/CB can also be applied jointly.
(2) The UL CoMP
For the UL CoMP, the UE receives the PDCCH from one transmission point, but Physical Uplink Shared Channel (PUSCH) transmission can be received concurrently by one or more transmission points. The UL CoMP is categorized in to Joint Reception (JR) and Coordinated Scheduling/Beam-forming (CS/CB). The JR refers to that a PUSCH transmitted by the UE can be received concurrently by all or a part of the points in the CoMP cooperating set to thereby improve the quality of receiving a signal. The CS/CB like the downlink CS/CB refers to that the points in the CoMP cooperating set are coordinated to be scheduled.
For the heterogeneous network with bearers being separated, since there may be a significant delay between the macro cell, and the small cell participating in bearer separation, there may be a limited gain of the CoMP applied between the macro cell, and the small cell participating in separating the bearers, but there may be a better delay, and a higher gain of the CoMP, between cells overlapping in coverage with the small cell participating in separating the bearers, and the small cell, so the CoMP may be applied between the small cell and the other cells to thereby improve the performance of the system.
However in the architecture with bearers being separated, all of RRC functions of the UE may be arranged in the macro cell, and both measurement configuration and measurement result reception is performed via RRC signaling, so the measurement configuration and the measurement result reception can only be performed in the macro cell, and even also CSI information can only be fed back in the macro cell. It is thus desirable to address the issue of obtaining a measurement result on the small cell for the purpose of the CoMP between the small cell and another cell overlapping in coverage area therewith (the macro cell where non-UE RRC connections are active).